It is well known in the prior art to frequency discriminate an input signal by multiplying this signal by a time-delayed version thereof and filtering the resulting product signal. The time-delay corresponds to a phase shift of the input signal which is dependent on input frequency, and the multiplication results in a signal having a d.c. component varying approximately linearly with the phase shift. To obtain the best resolution with good linearity for a given bandwidth, the time delay is accomplished using a delay line having its center frequency at odd multiples of 90.degree. phase. The delay line is also chosen to exhibit linear phase through the bandwidth of interest.
Although prior art frequency discriminator designs of the delay-line type have proven generally effective in certain applications, such designs have a major disadvantage in that the characteristics of resolution, bandwidth and linearity are all interrelated and fixed. Accordingly, while it may be possible to increase the resolution of a prior art frequency discriminator by a predetermined factor, such increase will cause a corresponding reduction in bandwidth by the same predetermined factor. Likewise, if the linearity of the design is increased by a predetermined factor, the bandwidth will likewise be reduced by this factor.
Accordingly, there is therefore a need for a method and apparatus for wideband frequency discrimination which minimizes the interdependence of the resolution, bandwidth and linearity characteristics of the discriminator and provides improved performance over prior art designs.